Transparent PNG texture casting shadows
Set the transparent bsdf to one color (solid white: 1, 1, 1...alpha is disregarded). For this shader to be truly "transparent," the color must be so.



Transparency color levels from left to right: 0, 0.25, 0.5, 0.75, and 1 (yes, there is an invisible monkey on the end).

Here is the demo blend:



And here is a link to the transparent shader in the manual:

https://www.blender.org/manual//render/cycles/nodes/types/shaders/transparent.html



The color can also be used to tint (it doesn't have to be the same for R, G, and B channels):



From left to right, the RGB color is as follows: (0.5, 0, 0), (0, 0, 1), (0, 0.75, 0), (0.75, 1, 0.75), (1, 1, 0).